A constant-velocity joint is a mechanical coupling which allows the shafts to rotate freely and compensates for the angle between the two shafts, within a certain range, to maintain the same velocity.
Tracta Joint
Rzeppa joint (compared to a 1 euro coin)
A diagram of a Thompson coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. In a more general context, a coupling can also be a mechanical device that serves to connect the ends of adjacent parts or objects. Couplings do not normally allow disconnection of shafts during operation, however there are torque-limiting couplings which can slip or disconnect when some torque limit is exceeded. Selection, installation and maintenance of couplings can lead to reduced maintenance time and maintenance cost.
An improvised flexible coupling made of car tyre pieces connects the drive shafts of an engine and a water pump. This one is used to cancel out misalignment and dampen vibrations.
A beam coupling
A beam coupling with optional features machined into it
Increasing number of coils allows for greater angular misalignment.